Abstract
The way an invasion progresses through space is a theme of interest common to invasion ecology and biological pest control. Models and mark-release studies of arthropods have been used extensively to extend and inform invasion processes of establishment and spread. However, the extremely common single-scale approach of monitoring initial spread leads to misinterpretation of rate and mode. Using the intentional release of a novel biological control agent (a parasitic hymenoptera, Eretmocerus hayati Zolnerowich & Rose (Hymenoptera: Aphelinidae), we studied its initial dispersal and spread at three different spatial scales, the local scale (tens of metres), field scale (hundreds of metres) and landscape scale (kilometres) around the release point. We fit models to each observed spread pattern at each spatial scale. We show that E. hayati exhibits stratified dispersal; moving further, faster and by a different mechanism than would have been concluded with a single local-scale post-release sampling design. In fact, interpretation of each scale independent of other scales gave three different models of dispersal, and three different impressions of the dominant dispersal mechanisms. Our findings demonstrate that using a single-scale approach may lead to quite erroneous conclusions, hence the necessity of using a multiple-scale hierarchical sampling design for inferring spread and the dominant dispersal mechanism of either human intended or unintended invasions.
Highlights
Invasion is a multi-step process comprised of three phases: initial dispersal; establishment of self-sustaining populations within the new habitat; and spread of the organism to nearby habitats [1,2,3]
Using the intentional release of a geographically novel biological control agent (a parasitic hymenoptera, Eretmocerus hayati Zolnerowich & Rose (Hymenoptera: Aphelinidae)) we studied its initial dispersal and spread at three different spatial scales: first, the ‘local scale’, which is of the order of tens of metres around the release point, second, the ‘field scale’, hundreds of metres around the release point, and third, the ‘landscape scale’, kilometres around the release point
If the local-scale adult parasitoid count had been the only data collected, from the shape of the distribution, one may have concluded that the data collection had captured the extent of the dispersal. Confidence in this conclusion would have been bolstered by the literature on related species E. eremicus (e.g. [46], [51]) for which dispersal has been observed confined to the field scale, and modelled using diffusion processes
Summary
Invasion is a multi-step process comprised of three phases: initial dispersal (where an organism moves from its native habitat, often over long distances, to a new habitat outside of its home range); establishment of self-sustaining populations within the new habitat; and spread of the organism to nearby habitats [1,2,3]. Biological control introductions are staged invasions where proliferation is managed by mass rearing and planned release with the hope of initial dispersal, establishment, and spread, and subsequent suppression of a target pest. Both human intended and unintended biological invasions have movement at the core of their success. Spread often occurs by stratified dispersal – the combined short- and long-distance movement, which are more often than not caused by completely different mechanisms [7,8,9,10,11,12]. There are several studies where the quantitative predicted rate of spread from diffusion models has been much lower than that observed [14], [15] or where transport by humans or wind is the most parsimonious explanation for rapid range expansion [7], [16], [17]
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